A satellite news gathering (SNG) system collects materials for broadcasting programs such as TV news by use of communication satellites. An earth station that collects the broadcasting materials is provided with a SNG vehicle and a flyaway (transportable earth station that is not vehicle-mounted). For such an SNG system, a solid state power amplifier (SSPA) incorporating a field-effect transistor (FET) is often adopted as a power amplifier to amplify transmission signals.
The SNG system generally uses the C band (4 to 8 GHz) or the Ku band (12 to 18 GHz) for satellite communications. For such frequency ranges, gallium arsenide FETs (GaAsFETs) are most commonly employed as power amplifying FETs. For example, because a GaAsFET obtains a gain of approximately 6 dB in the Ku band, multiple FETs need to be cascaded in order to achieve a gain as high as 60 dB that is required for the SNG system. When multiple FETs are cascaded, a compensating function is required to maintain the gain of the FETs constant because the gain tends to vary in accordance with the ambient temperature.
For instance, Jpn. Pat. Appln. KOKAI Publication No. 2007-82016 (paragraph 0014, FIG. 1) teaches a technology of arranging temperature sensors in the vicinity of a main amplifier circuit and a distortion signal amplifier circuit to monitor the temperatures of heater elements of the circuits and elements whose properties greatly change in accordance with the temperature. According to this technology, the gain properties of each element in relation to temperatures are stored in advance on a look-up table, and the table is used for temperature compensation.
When multiple FETs are cascaded in a SSPA, however, a small amount of heat is generated by the FETs on the first part, while a large amount of heat is generated by the FETs on the later part. This means that the temperature is not distributed evenly inside the SSPA. In addition, with a single temperature sensor provided in the vicinity of the amplifier circuit to detect the temperature at a single position, the gain variation incurred by a temperature change at any position other than the temperature detection position cannot be corrected.